This patent application relates to driver circuits for laser diodes, and more particularly, to driver circuits that suppress jitter in tone modulated single ended output signals.
Laser diodes are used to convert electrical signals to optical signals in fiber optic communication systems. A laser driver circuit provides a current signal to drive a laser diode. The current signal contains modulated data. The laser diode outputs an optical signal that contains the modulated data.
A power control feedback loop controls the output signal of the laser diode. The power control feedback loop includes a photodiode. The photodiode monitors the optical power of the laser diode output signal and provides an electrical signal indicative of the laser diode output signal.
The power control feedback loop compares the photodiode output current with a reference signal. In response to this comparison, the power control feedback loop tunes the output signal of the laser driver dynamically to control the optical power of the laser diode.
Recently, there has been a drastic increase in the data rate used in laser diodes (especially in the OC192 range). The photodiode in the power control feedback loop cannot respond to a very fast data rate. The bandwidth of photodiodes is typically in the range of 1 MHz. However, the output signal of a laser diode operated at a fast data rate may, for example, be around 5 GHz.
Therefore, it is desirable to have a low frequency component of the laser diode output signal that can be sensed by the power control feedback loop. In order to provide a frequency component of the laser diode output signal that is within the bandwidth of the photodiode, a low frequency tone signal can be added to the laser driver output current. The tone frequency is low enough so that it is within the bandwidth of the photodiode. The tone signal allows the power control feedback loop to regulate the laser diode output signal, even when the laser diode output signal is otherwise outside the bandwidth of the photodiode.
However, at least one significant problem is caused by adding the tone signal to the laser driver output current. The tone signal can create extra jitter (i.e., unwanted noise signals) in the output signal of the laser driver circuit. This is true even though the tone signal has a very low frequency (e.g., 100 KHz).
Laser driver circuits typically use single-ended current output signals to drive laser diodes. It is more challenging to control jitter in a single ended signal than in a differential signal. Jitter in a laser driver becomes more serious when tone modulation is added to the laser driver.
The low frequency tone signal will cause a considerable amount of extra jitter even in a clean single ended signal. This extra jitter is not an issue for differential signals, because the zero crossing point does not change.
Therefore, it would be desirable to reduce jitter in laser driver circuits at the zero crossing points of the output signal.